How heat waves fuel Arizona's wildfire risk

The link between heat and wildfire is well known to climate researchers like Dmitri Kalashnikov, a postdoctoral scholar at University of California-Merced.

"If it's hot weather, it's more likely to be the kind of conditions that enable wildfires to burn,” Kalashnikov said. It’s one of three ingredients that make up fire weather: a volatile combination of temperature, wind and humidity. Hotter temperatures can suck water out of the atmosphere, sapping humidity levels and drying out plants.

Those hot days are happening more often, especially in the West. This year, Arizona reached triple digits during an early heat wave in March.

“The Western U.S., along with the rest of the world, is warming,” Kalashnikov said.

Summer temperatures in the region have warmed about 2 degrees in the last 25 years. Because of those warmer conditions, we see more heat waves than we used to, he said. So while a century of fire suppression and decades of drought are fostering bigger and more severe wildfires, climate change has become a key driver of the West’s wildfire problem, according to the National Oceanic and Atmospheric Administration.

But the effect of consecutive extreme heat days on wildfire risk has never really been quantified. Kalashnikov and his research team wondered just how much the West’s increasingly frequent heat waves are impacting its wildfire season.

It turns out, a lot.

Heat waves fuel growing risk of wildfire

In the U.S. the number of wildfires each year has stayed fairly constant over the last four decades, according to the National Interagency Fire Center — but the amount of burned area from wildfires each year has increased.

Fires like those sparked in Washington and Utah this summer have spread rapidly over tens of thousands of acres. In Beaver County, Utah, a fire burned to about the size of Mesa over the course of a month. In Spokane, Washington, a trio of wildfires surrounded residential neighborhoods on the outskirts of the city at the beginning of August. In both states, the fires were amplified by “red flag” fire weather characterized by the severe conditions that are becoming more and more common.

Since 2001, the number of heat wave days — defined as three or more consecutive days of temperatures in the top tenth of hottest days — across Western U.S. forests has nearly doubled. During that same period the amount of forested land burned by wildfire increased by 2.5 times.

“We see an increase in burned area during and immediately following heat waves mostly everywhere, especially during the actual heat wave,” Kalashnikov said. “That response is seen all across the West, whether you're in the drier areas in the Southwest or cooler areas further north.”

From 2001 to 2024, wildfires that started during or right after a heat wave accounted for about 42% of all the area burned, even though those extra hot days only accounted for less than a fourth of warm-season days in general.

How do heat waves raise wildfire risk?

While typical fire weather forecasts consider temperature, Kalashnikov said they usually only factor in daily temperatures as opposed to tracking repeated highs. Understanding heat waves as a primary driver of wildfire risk can help forecasters better anticipate when a fire might ignite and how quickly it could move.

“Multi-day heat events are important because you have this compounding effect,” he said.

During a heat wave, hot weather conditions don’t always dissipate when the sun goes down; they can last through the night.

“On a typical night, in a lot of places, it cools off quite a bit,” Kalashnikov said. When it cools off, the humidity rises, the air is more moist and fires tend to calm down. Without that cooling effect, fires stay active.

“Essentially, think of it as more hours in the day available for burning compounded over several days and nights,” he said. That enables a fire to continue spreading.

As the atmosphere heats up during a heat wave, the researchers also found increases in dry lightning, which can occur when rain from a storm evaporates before it hits the ground. Lightening frequently sparks wildfires, but without the rain associated with a thunderstorm the flames can spread unchecked.

Across the West, there are a few key differences in the ways heat waves can influence wildfire depending on the specific region. As temperatures rise, relative humidity, which is the amount of water vapor in the air compared to how much it could hold at a given temperature, decreases, according to Climate Central, a research nonprofit.

But a drier heat wave has a much stronger influence on wildfire activity than a more moist heat wave, Kalashnikov said.

“If the heat wave has relatively high humidity, it actually doesn't increase the burned area because the air is moist enough to mitigate the fire risk,” he said. “It's really those dry heat waves that matter.”

In the thick of summer, dry heat waves are more common in the northern and western parts of the region — like in Spokane. In the Southwest, they tend to dissipate with the arrival of the monsoon.

Climate change makes heat waves more likely

As a heat dome builds over the Four Corners this weekend, the National Weather Service warns it could bring dangerously hot temperatures to much of the West.

Some models show that the forecast temperatures in Arizona, Southern California, Nevada, Utah and Colorado are made at least five times more likely by climate change.

While we can’t know that climate change causes specific extreme weather events, researchers are increasingly able to study how climate change can make events like heat waves more severe or more likely to occur. It’s called attribution science, and it examines the links between global warming and human interference.

 ”We have gained a greater ability to more rapidly attribute certain events and developments to changing climate,” said Max Boykoff, a professor of environmental studies at the University of Colorado Boulder.

Climate Central tracks the number of days where temperature has likely been affected by climate change using its Climate Shift Index. As of July 29, a total of 107 days in Phoenix this year had temperatures made twice as likely by climate change.

As the region gets warmer, its fire season expands. In the last 50 years, Arizona, New Mexico, Southern California and Texas have seen some of the greatest increases in annual fire weather days, according to Climate Central. In Southeast Arizona, annual fire weather days have gone up by 57 days since the 1970s.

That’s more than an additional month of increased wildfire risk.

For Kalashnikov, the connection between heat and fire is clear.

“Given how things are going,” he said, “the expectation is that we will continue to see more heat waves and increased burned area, especially in the forests of the Western U.S.”